If
a man is not a good husband, is it biblically acceptable for his wife to
leave him, or to threaten him with divorce in order to force him to
change?

An
answer to this question can be deduced based on the Bible’s principles
related to marriage and divorce. For a wife to go against her husband's
desires and leave him, would require that she forsake her biblical
responsibility to submit to his authority. Such a wife may justify her
separation by claiming she is simply following Christ's example -- he
set boundaries and "drew lines" of behavior for his followers,
and that is what she is doing for her husband. But such a wife
misconstrues her calling to be Christ-like -- she is called to be like
Jesus in character, not in authority.
In fact, the Bible tells her specifically that she is not to be like Christ
in the exercise of authority, but to be like the Church
who submits to Christ's authority (Eph 5:24).

A
woman may even claim she is simply being a helpmate
trying to help her husband
repent. However, God does not allow those under
authority to change the ones in
authority by disobeying them. A wife, therefore, has no biblical
permission to act outside her husband's authority, or to rebel against
him, even if her intention is to coerce him to repent. Quite to the
contrary, God specifically commands the wives of disobedient husbands to
behave with respect and submissiveness toward them, which He says can
win them over. Speaking through Peter, God tells women, "Wives,
in the same way be submissive to your husbands so that, if any of them
do not obey the word, they may be won over without words by the behavior
of their wives, 2 when they see the purity and reverence of your lives.
3 Your beauty should not come from outward adornment, such as braided
hair and the wearing of gold jewelry and fine clothes. 4 Instead, it
should be that of your inner self, the unfading beauty of a gentle and
quiet spirit, which is of great worth in God's sight. 5 For this is the
way the holy women of the past who put their hope in God used to make
themselves beautiful. They were submissive to their own husbands, 6 like
Sarah, who obeyed Abraham and called him her master. You are her
daughters if you do what is right and do not give way to fear."1 Peter 3:1-6NIV/NRSV

The wife who threatens to divorce her husband "unless he
changes," should consider that her ultimatum places her
in charge of the relationship. No longer is she under his authority -- by her demands she assumes his position of leadership and attempts to take control of him. If
her exertion of authority is submitted to by her husband, he becomes
accountable to her, and she presides as the judge of his sincerity,
sitting in constant evaluation of his performance. She has made
fulfillment of her own wifely duties conditional upon her husband's
behavior, and at some point must give up her superior role and resubmit
herself to his authority, until such time that she deems it necessary to
reassume control and force him to repent again. Although such a wife may
succeed in coercing her husband to wake up and begin working harder to
please her, she will find herself unable to have a balanced, biblical
relationship with him, because she remains the one in control. Can a
healthy, biblical marital relationship be gained by venturing outside
biblical marital roles? Should that wife expect to enjoy God's blessings
when she resists His plans so strongly!?She most definitely will not! Those who believe that acts of
"tough love" can successfully stir husbands to genuine
repentance may see some change, but the fruit born will not be of
eternal quality.

Some wives are convinced that to abide by God's command here
would only make matters worse. They claim they have tried to be the
"1 Peter 3" wife, but did not see fruit. But is that possible?
Can God's Word be wrong?"Let
God be true, and every man a liar"! (Rom 3:4). God is never
wrong. If He says that the best way for a wife to win her husband's
heart is through humility, submissiveness, and respectful reverence,
then God knows what He is talking about! If such godly behavior has not
born fruit, then we may not conclude that God's solution is inadequate.
Either that wife has not actually followed Sarah's example, so her
husband has not responded, or she has been the model of humility and
respect, but has not waited long enough for the fruit to be born.
Whatever the reason for her husband's continued hardness, we know that
in 1 Peter 3 God has given His best plan. Since the Bible reveals God's
wisdom, then the plans we devise with our
wisdom will be inadequate and possibly dangerous. No matter how wise we
perceive ourselves, or how intense our emotions, we cannot create a
better way than God's to accomplish spiritual goals. In the words of the
apostle James, "… man's
anger does not bring about the righteousness of God," (James
1:20). Righteous ends cannot be attained by fleshly efforts. Those who
trust in the flesh will be disappointed (Jer 17:5; Phil 3:4).